20;0 LAND REGIONS. 



Brazilian forms are those in which tlie aperture is dentate, 

 angulated, or auriculiform. 



26. Mexican Region. 



Includes the Republic of Mexico and Central America ; Lower 

 California also belongs to it zoologically. This extensive country 

 comprises three physical regions ; the comparatively rainless and 

 treeless districts of the west ; the mountains or high table-lands 

 with their peculiar flora ; and the rainy wooded region that 

 borders the Caribbsean sea. It is divided by Fischer into four 

 geographical subregions : 1. Subregion of the Gulf, comprising 

 the coasts of the Gulf States (Tamaulipas, Vera Cruz, Tabasco, 

 Campeche, Yucatan), the isthmus of Tehuantepec and Central 

 America; 2. Subregion of the centre, formed of the temperate 

 and cold districts (Puebla, Mexico, Zacatecas, etc.) ; 3. Subre- 

 gion of the Pacific ; 4. Subregion of Lower California. 



The Subregion of the Gulf is characterized by the genera 

 Strebelia, Streptostyla, Petenia, Glandina, Xanthonj^x, Helix 

 (groups Odontura, Geotrochus, Corasia), Coelocentrum, Eucalo- 

 dium, Cylindrella, Macroceramus, Orthalicus, Bulimulus, Opeas, 

 Spiraxis, Subulina, Yaginula, Aplecta. All the Mexican oper- 

 cul'ated land snails inhabit this svibregion : Cyclotus, Cyclophorus, 

 Tomocyclus, Choanopoma, Adamsiella, Tudora, Cistula, Chon- 

 dropoma, Helicina (36 species) ; as well as the genera Ceres 

 and Proserpinella of the family Proserpinidse : most of these 

 genera are West Indian types. The Melanian genus Pachy- 

 cheilus is peculiar to the region and its metropolis is Central 

 America. A number of Ampullaria occur in the southern portion, 

 as well as a single species of the South American genus Hemisinus, 

 in Guatemala. There are numerous fine species of Unionidae 

 and a few Cyrenidse. 



The colder central region contains a very much less extensive 

 fauna. There are a few Glandina, a Helix of the Pomatia section, 

 H. Humboldtiana, and several small species of the Polygyra 

 group ; Holospira replaces the Cylindrella and Eucalodium of 

 the warmer regions. A single Melanian, and a few Physa, 

 Limngea and Planorbis constitute the fluviatile gastropods. 



The Pacific subregion is rather bare of mollusks also, but 

 includes some characteristic fluviatile forms ; the large Aplecta 

 aurantia, the only North American Corbicula, 4 Cyrena, etc. 



In Lower California the existence of 12 species of Bulimulus 

 shows the aflflnity of the fauna with that of Mexico and of 

 Western South America; on the other hand the Helices, few in 

 number, are of Californian types. Berendtia is restricted to 

 this subregion ; the paucity of its species corresponds to the 

 desert nature of its soil. 



The Mexican fauna is characterized by the existence of num- 



